Sensor

ABSTRACT

An inertial sensor is described that has a commanded test function. The sensor is of a ring type driven by a driver circuit, the sensor further includes primary and secondary portions having corresponding signal pickoffs. The primary pickoff signal amplitude is controlled via an automatic gain control, the primary phase lock loop and VCO locks to the resonant frequency to provide the clocks for the synchronous detectors, the primary pickoff signals via the primary phase shift circuit is provided to the primary driver, the secondary pickoff signal being input into a detector circuit capable of detecting motion in the sensor. The commanded test function includes a signal derived from the primary portion of the circuit and input into the two inputs of a differential amplifier in the secondary pickoff detector circuit.

CROSS REFERENCE TO RELATED APPLICATION

The present application is the U.S. national stage application ofInternational Application PCT/EP2011/053184, filed Mar. 3, 2011, whichinternational application was published on Sep. 9, 2011, asInternational Publication WO 2011/107542. The International Applicationclaims priority of British Patent Application 1003539.2, filed Mar. 3,2010, the contents of which are incorporated herein by reference intheir entireties.

The invention relates to sensors. More specifically but not exclusivelyit relates to sensors such as inertial sensors, for exampleaccelerometers and gyroscopes, having a commanded built in test (CBit).

Many types of inertial sensors are known. Angular velocity sensorsincorporating a MEMS type ring are known and such examples can be seenin, for example, GB2322196. In such angular velocity sensors a vibratingplanar ring or hoop-like structure is disclosed. The ring-like structureis suspended in space by a suitable support mount for detecting turningrate, linear acceleration and angular acceleration. Turning rate issensed by detecting vibrations coupled by Coriolis forces, whereaslinear acceleration and angular acceleration are sensed by lateral,vertical and rocking movement of the entire ring or hoop-like structurewithin its mount.

The ability for a user to command a check of a MEMS rate sensor haspreviously been limited to a connectivity check. A typical MEMS ratesensor user check either just forces the MEMS structure to end limits orthe tolerances of an in range change are such that limited informationof operation can be achieved. Also these checks often provide verylimited coverage of the complete system operation.

According to the invention there is provided an angular velocity sensorcomprising a ring type sensor having primary and secondary elements, thesensor further comprising primary and secondary channels connected tosaid primary and secondary elements, the primary channel comprisingprimary driver means for initiating and maintaining resonantoscillations in the primary elements, the secondary channel comprisingdetector means for detecting signals generated by the secondary elementsin response to movement of the sensor, the detecting means including adifferential amplifier and synchronous detectors generating an outputsignal dependent upon the movement of the sensor, wherein the secondarychannel further comprises test means that, when operable, inputs signalsonto both channels of the secondary channel differential amplifier, saidsignals being respectively out of phase such that an offset in theoutput signal is generated, said offset being indicative of the statusof the sensor.

The invention will now be described with reference to the accompanyingdiagrammatic drawings in which:

FIG. 1 is a schematic drawing of an angular sensor in accordance withone form of the invention including commanded built in test (Cbit)enabling testing of primary MEMS pickoff transducers, primary pickoffamplifier, secondary pickoff amplifier, primary automatic gain control(AGC) loop, primary phase lock loop and voltage controlled oscillator,primary drivers, primary MEMS driver transducers and primary channelpickoff phase shifter (PP90); and

FIG. 2 is a schematic diagram of one form of the Cbit signal injectioncircuit into pickoff amplifiers of the angular velocity sensor of FIG.1.

As shown in FIG. 1, the MEMS ring structure 1 of the angular velocitysensor comprises inner 2 and outer 3 peripheries extending around acommon axis and therefore the drive and pickoffs operate differentially.Piezoelectric primary drive means 4 are provided that cause the sensorto vibrate on supports (not shown). The supports may include a pluralityof flexible support beams (not shown) for supporting the sensor and forallowing it to vibrate in response to the piezoelectric primary drivemeans input in a substantially undamped oscillation mode. This enablesthe resonator to move relative to the support means in response toturning rate of the system including the sensor.

A detecting circuit 5 is provided to which a series of signals 6 areinput from the sensor. The signals output by the sensor include primarypickoff signals 6 a and secondary pickoff signals 6 b and the transducerplates are arranged such that the signals are of a differential form.These signals are output from primary and secondary portions of thesensor. The primary pickoff differential transducer signal is input to adifferential charge amplifier (primary pickoff amplifier) 7 whichprovides a high level of gain to provide a low noise differentialsinusoidal output signal at the carrier frequency. This signal is thenpassed through a synchronous detector 8 and suitable filtering toprovide a control signal to the primary driver circuit 4 to set thelevel of the drive applied to the MEMS ring structure 1 to ensure theprimary pickoff amplifier output is on a controlled fixed level. Theoutput of the primary pickoff amplifier 7 is also applied to the primaryphase lock loop (PLL) and voltage controlled oscillator (VCO) 10 whichlocks to the pickoff signal to provide clocks for the synchronousdetectors 8. The output of the primary pickoff amplifier 7 is alsoapplied to the primary phase shift circuit 11 which amplifies the signalby a factor of two and shifts the phase of the differential signal by90°. The output of the phase shift circuit 11 is applied to the primarydriver circuit 4 which multiplies the differential sinusoidal output ofthe phase shift circuit 11 by the AGC control signal to generate thedifferential MEMS driver waveforms. The output of the phase shiftcircuit 11 is applied to the commanded BIT (CBit) injection circuit 12which provides a high level of isolation when CBit is not selected andcouples in an attenuated version of the differential signal to thedifferential inputs of a further differential charge amplifier(secondary pickoff amplifier) 13 when commanded BIT is selected. Theinjected differential signal is then amplified by the secondary pickoffamplifier 13 and is of the correct phase to pass through the ratechannel synchronous detector 14. The synchronous detector outputs anoffset relative to the amplitude of the injected differential signalwhich is then filtered and converted into a single ended offset on therate output signal and input to the ADC 15.

The requirement of Cbit is to test as much of the system as possible toprovide a relatively known output. Under the control of an enablingsignal, the differential primary pickoff signal with a 90 degree phaseshift can be fed into the differential inputs of the secondary pickoffamplifier 13, not the pick offs which are physically on the ring. Thissignal is of the correct phase to be demodulated and used to provide acontrolled output of the rate output. This tests the primary pickoffMEMS transducers, the primary pickoff amplifier 7, the primary AGC loopthe primary PLL and VCO channel 10, the primary driver circuits, theMEMS primary driver transducers, the primary phase shift circuit (PP90)11 as well as the secondary pickoff amplifier 13, rate channel 14, rateoutput and ADC 15.

As the secondary pickoff amplifier 13 has a high gain it is necessary toattenuate the signal to avoid saturation of the rate channel 14. Inaddition, the rate setting used in the rate output stage is not known,so a smaller signal is used to avoid saturation of the rate output.

FIG. 2 shows the method used in more detail. The signal is added at thesumming junction on both of the differential inputs to the secondarypickoff charge amplifier 13. It is coupled in by a small capacitor Cinj(260 fF) to create a first stage gain of 0.007 (Cfb=35.8 pF). Along witha 5:1 input attenuator (Ri1=200 k, Ri2=50 k) and a second stage gain of35, the overall gain is 0.05. Thus a 500 mV peak signal on primary phaseshift circuit (PP90) output creates a secondary pickoff signal of 25 mVpeak.

With Cbit enabled, a signal appears on the output of the secondarypickoff amplifier 13 in phase with the rate channel demodulator clock.In this mode, an offset of around 50 deg/sec is seen on the rate outputsignal, both on the analogue signal and the digital signal. This offset,when OBIT is operative, demonstrates correct operation of the gyro tothe user.

In this way, the user command test function (Cbit) uses signals from theprimary resonant sustaining loops to generate a disturbance in thesecondary rate detection channel.

On application of the test the correctly phased signal from the primaryresonant sustaining loop is coupled into the front end of theelectronics secondary rate detection channel, scaled to a suitablelevel. This passes through the complete rate detection channel andprovides a measurable change in the output signal of the usual ratesignal.

As the disturbance is generated from the differential MEMS transduceroutputs of the primary resonant sustaining loop it ensures that thesystem has locked to the resonant frequency and that the resonantsustaining loop amplitude of motion is controlled correctly.

As the disturbance is coupled into the very front stage of theelectronics rate detection channel the complete rate detection channelis checked (excluding the secondary pick off transducers on the ring).

The tolerances are such that the magnitude of the rate signaldisturbance can be used to detect loss of part of the circuitry in boththe primary resonant sustaining loop and the secondary rate detectionchannel although it may not identify which circuit element has failed.

When disabled, the Cbit injection path has a high level of isolation bythe use of series and parallel switches (Rmax_on=1 k). Additionally theoutputs of the injection block have 50 ohm series resistors to providecurrent limiting (for ESD protection) as the summing nodes are directlyconnected to the pickoff bond pads.

In order for Cbit to give a correct offset then the phase lock loop andVCO 10 part of the primary loop must be locked to enable the primarydifferential pickoff signals to exist. The AGC 16 must be operating togive the correct amplitude and therefore correct level of primarypickoff amplitude. The primary pickoff and the primary phase shiftcircuit 11 must be operating to provide the primary drive signals andthe differential signal through the Cbit injection circuit 12 to thesecondary loop. The primary drivers must be operating to close the loopsaround the primary circuit and the secondary pickoff amplifier 13 mustbe functioning to give the correct level at the rate channeldemodulator. The rate channel demodulator, filtering, decimator andnotch filter must be correct to provide correct level to the rate outputstage. Additionally, the rate output gain setting must be correct toachieve the expected offset on the output and finally the ADC 15 and ADCreferences must be operating correctly to achieve the correct offset onthe digital rate output.

In a second embodiment of the invention, the operation of the primaryand secondary channels is broadly the same, the primary channel includesa synchronous detector 8 receiving a signal from the primary pick off onthe ring, the output of which feeds back to the primary drive, and iscontrolled in amplitude by an AGC. This gives closed loop operation ofthe primary loop. The secondary channel operates in open loop, so thereis no feedback.

In the primary loop there is a phase locked loop to control thefrequency of the primary loop by means of a VCO. The secondary channelpicks up the Coriolis coupled signals at the secondary transducers onthe ring. There are electrodes on the inside and outside part of theannular ring which give signals in anti phase. These anti phase signalsare injected to two inputs of a differential amplifier 13 the output ofwhich feeds to the secondary synchronous amplifier 14 and thence to therate output. Also the signals at 45 deg and 225 deg (secondary pickoffs) on the ring are anti phase to the signals at 135 and 315 deg. Thisassumes that the primary has pick off and drive transducers which may beat 0, 90, 180 and 270 degrees Thus we have 4 secondary pick off in phaseand 4 pick offs anti phase in as shown in FIG. 1 (4 electrodes on theoutside and 4 on the inside of the annulus). The two anti phase signalsgo to the two inputs of the differential amplifier of the secondarychannel charge amplifier 13 into which the OBIT signal is injected whichis derived from the primary signals. This then goes to the secondarysynchronous amplifier and thence as the rate output signal. The CBITinjected signal thus causes an offset of the rate output signal, ratherthan there being a separate test output channel.

In a third embodiment of the invention, in which all similarly numberedparts have the same function as those described above, two signalsderived from the primary channel are injected via a suitable attenuationcircuit onto the two inputs of the secondary channel differentialamplifier 13 virtual earth points under command of a signal from theuser. Both signals are at 180° phase with respect to the 0° phaseprimary drive waveform. The first signal is connected to one of thesecondary pickoff differential amplifier 13 inputs and the second signalis connected to the other input of the differential amplifier 13. Theamplitudes of the respective signals are different. The differentialamplifier thereby produces an output which is related to the differencein the amplitude of the two applied signals. This difference will passthrough the rate channel synchronous detector 14 and cause a usermeasurable change in the rate signal. Suitable choice of the differencein the amplitudes of the two applied signals will allow detection ofsingle input channel failures as well as the correct operation of theprimary and secondary channels.

In a fourth embodiment of the invention two signals derived from theprimary channel are injected via a suitable attenuation circuit onto thetwo inputs of the secondary channel differential amplifier 13 virtualearth points under command of a signal from the user. The signals are at45° and 225° phase with respect to the 0° phase primary drive waveform.The first signal is connected to one of the secondary pickoffdifferential amplifier 13 inputs and the second signal is connected tothe other input of the differential amplifier 13. The differentialamplifier 13 will then produce an output which is related to thedifference in the amplitude of the two applied signals. A proportion ofthe applied signal that is in phase with the rate channel synchronousdetector 14 clock will pass through the rate channel synchronousdetector 14 and cause a user measurable change in the rate signal. Aproportion of the applied signal that is in phase with a quad channelsynchronous detector 18 clock will pass through the quad channelsynchronous detector 18 and cause a change in the quad channel signal.This will allow detection of single input channel failures as well asthe correct operation of the primary and secondary channels. In additionthis allows the quadrature channel function to be check if desired.

In a fifth embodiment of the invention two signals are derived from foursignals from the primary channel, said two signals being injected via asuitable attenuation circuit, onto the two inputs of the secondarychannel differential amplifier 13 virtual earth points under command ofa signal from the user. The four signals are at −90°, +90°, 0° and 180°with respect to the 0° phase primary drive waveform. These are generatedfrom two parts of the primary circuit. This creates a 45° and a 225°signal where one signal is connected to one of the secondary pickoffdifferential amplifier 13 inputs and the other signal is connected tothe other input of the differential amplifier 13. The differentialamplifier 13 will then produce an output which is related to thedifference in the amplitude of the two applied signals. A proportion ofthe applied signal that is in phase with the rate channel synchronousdetector 14 clock will pass through the rate channel synchronousdetector 14 and cause a user measurable change in the rate signal. Aproportion of the applied signal that is in phase with the quad channelsynchronous detector 18 clock will pass through the quad channelsynchronous detector 18 and cause a change in the quad channel signal.This will allow detection of single input channel failures as well asthe correct operation of the primary and secondary channels. In additionthis allows the quadrature channel function to be check if desired. Alsoas the signals are generated from two points of the primary channel itprovides additional fault detection of the primary channel.

A further embodiment of the invention will now be described where CBitis implemented using a circuit similar to that described in FIG. 1 andin which all similar components performing the same function aresimilarly numbered. In this embodiment the circuit provides an offset atthe output of the secondary pickoff amplifiers 13, when commanded, thatis at 45° with respect to the rate 14 and quadrature demodulator clocks.The offset therefore contains a component of the signal that is both inphase with the rate channel demodulator clock, and therefore provides anoffset through the rate channel which can be seen on the rate signal,and a component of the signal in phase with the quadrature demodulatorclock, therefore provides an offset through the quadrature channel whichcan be seen on the quadrature signal.

The 45° offset is generated by the following method. The 0° phaseprimary pickoff signal is applied with suitable attenuation to one ofthe inputs of the differential secondary pickoff amplifier 13. The 90°phase shifted primary pickoff signal is applied with suitableattenuation to the other input of the differential secondary pickoffamplifier 13. The secondary pickoff amplifier then mixes the two signalsand generates the 45° output signal. In this embodiment, the connectionsto the Cbit injection circuit 12 are from two different points withinthe primary channel and the quad channel now feeds an ADC using eitherone or two ADC circuits not shown.

By implementing the circuit in the above described way it providesadditional fault coverage, over and above the previously described Cbitimplementation.

For example, the circuit can detect a loss of a wire bond connectionfrom the MEMS secondary pickoff transducers 2 to the secondary pickoffamplifier. A loss of a wire bond causes the gain on that channel of thedifferential secondary pickoff amplifier to change. This results in theeffective phase of the output offset under Cbit changing which will thenchange the level of the offset measure on the rate 14 and quadraturechannels.

Secondly, the quadrature signal is now used as part of continuousmonitoring of the health of the system. The application of the 45° Cbitsignal allows the quadrature monitoring channel operation to be checkedto ensure correct operation and gain in addition to the rate channel 14.

Thirdly, the circuit that generates the 90° phase shifted pickoffsignal, which is used elsewhere in the primary channel of the systems,also has additional level of fault detection. As the Cbit injection usesone signal from before this circuit and one signal from the output ofthis circuit a gain and phase error within this circuit will cause theCbit offset to be incorrect and therefore detectable in a change in thelevels on the rate 14 and quadrature channels.

Moreover, using the adapted circuit, causes a gain change, due to aninternal circuit fault, on one channel of the secondary pickoffamplifier 13 circuit and would cause the phase and offset of the Cbitsignal output to alter and therefore is detectable on the quadrature andrate 14 channel offsets.

It will be appreciated that one further aspect of all of theimplementations described above, which applies to both the normal Cbitand the 45° Cbit variants, is that where there is more than oneselectable output measurement range, the offset levels are deliberatelychosen to be different. This allows the incorrect range selection to bedetected as the offset under application on the Cbit signal would bedifferent to the level expected.

It will be appreciated that although the invention is described withreference to an annular velocity sensor of the type described above, itmay equally be applied to a sensor such as a piezo ring case, the sameapplying to other forms of transduction such as piezo, capacitive,inductive.

It will further be appreciated that the inner and outer electrodes onlyrelate to the piezo. Anti phase signals can be obtained from 45 deg to135 deg etc from non piezo pick offs, such as inductive or capacitivepickoffs.

The invention claimed is:
 1. An angular velocity sensor comprising aring type sensor having primary and secondary elements, the sensorfurther comprising primary and secondary channels connected to saidprimary and secondary elements, the primary channel comprising primarydriver means for initiating and maintaining resonant oscillations in theprimary elements, the secondary channel operating in an open loopmanner, thereby requiring no feedback and the secondary channelcomprising detector means for detecting signals generated by thesecondary elements in response to movement of the sensor, the detectormeans including a secondary channel differential amplifier andsynchronous detectors generating an output signal dependent upon themovement of the sensor, wherein the secondary channel further comprisesselectively operable test means that, when operable, inputs signals ontoboth channels of the secondary channel differential amplifier, saidsignals being respectively out of phase such that an offset in theoutput signal is generated, said offset being indicative of the statusof the sensor.
 2. A sensor according to claim 1 in which the primaryelements generate two primary pickoff signals output to the primarychannel, said primary pickoff signals being amplified and phase shiftedby suitable means and input into respective channels of the secondarychannel differential amplifier.
 3. A sensor according to claim 2 inwhich the phase shift is 90 degrees.
 4. A sensor according to claim 1 inwhich the outputs of the secondary channel differential amplifier areinput to a rate channel synchronous detector and a quad channelsynchronous detector, said rate channel synchronous detector having aclock input, the output of the secondary channel differential amplifier,when the test is operable, generating a signal in phase with the ratechannel clock.
 5. A sensor according to claim 1, in which the primaryelements comprise pick off and drive transducers at 0, 90, 180 and 270degrees on the ring and the secondary elements comprise 45, 135, 225 and315 degrees.
 6. A sensor according to claim 5 in which signals generatedby the 45 and 225 degree elements on the ring are anti phase to thesignals at 135 and 315 degrees.
 7. A sensor according to claim 1 inwhich the elements are piezo electrodes.
 8. A sensor according to claim1 in which the signals applied to the secondary channel differentialamplifier are respectively 180 degrees out of phase.
 9. A sensoraccording to claim 1 in which the primary channel includes a synchronousdetector receiving a signal from the primary pick off on the ring, theoutput of which feeds back to the primary drive, and is controlled inamplitude by an automatic gain control (AGC), thereby enabling closedloop operation of the primary channel.
 10. A sensor according to claim 1in which the primary channel further comprises a phase locked loop tocontrol the frequency of the primary channel by means of a voltagecontrolled oscillator (VCO).